Addition of titanium carbide-based materials significantly reduces friction and wear
Anywhere there are surfaces in relative motion, there will be friction and wear to contend with. And they come at an astonishing cost. Almost a quarter (23%) of all energy consumed worldwide (119 EJ) originates from tribological contacts. The vast majority (20%) goes towards overcoming friction, with the remaining 3% used to remanufacture components lost due to wear and wear-related failures. Finding ways to control and reduce friction is therefore a global priority. Complex lubricant formulations, coupled with direct surface modification techniques (e.g. coatings and plasma treatment) have been hugely successful in many applications. There is also a growing interest in using composite materials that have been specifically designed to have optimised tribological properties.
A paper from scientists at India’s CSIR-AMPRI has added some new knowledge to this effort. Writing in the latest issue of Carbon [DOI: 10.1016/j.carbon.2024.118790], they report on a polymer composite that displays low friction, reduced wear, and damage-healing capability. Their composite uses a shape-memory polyurethane (SMPU) as the model polymer matrix and titanium carbide-based MAX and MXene materials as fillers.
MAX materials are functional ceramics – typically based on ternary carbide or nitride – with a layered structure. They exhibit high mechanical strength and favourable electrical and thermal conductivities. MXenes, which are 2D materials exfoliated from the MAX phase, retain many of those same properties. In addition, they are hydrophilic, which allows them to form strong bonds with a range of matrix materials, and they have low shear strength. These characteristics suggest that, as fillers, these materials could improve the wear resistance and frictional performance of composites.
For this study, the team chose two MAX phases – Ti2AlC (MAX1), and Ti3AlC2 (MAX2) – and one MXene, Ti3C2, and employed filler concentrations of 0.25, 0.5, 1.0, and 2.0 wt%. A sample of pristine SMPU was also produced, and acted as a reference.
To evaluate the sliding friction and wear properties of the composites, the researchers employed a ball-on-disk tribometer. In these tests, the friction coefficient of the SMPU was seen to be initially similar to that of the SMPU-MAX1, SMPU-MAX2, and SMPU-MXene composites. However, beyond ~700 cycles, the friction coefficient of the SMPU increased considerably, reaching an average of 0.35. The authors credit this to the onset of frictional heating in the polymer, which led to deformation and increased adhesion forces. In contrast, the average friction coefficient of all composites stayed below 0.13, at all filler loadings. In fact, the percentage loading had no significant impact – similar friction reduction was seen for 0.25 wt% and 2.0 wt% composites.
Images of each sample and the ball probes were collected following the tribometer tests. They conformed the presence of a large wear track on the pristine SMPU samples, and significant transfer of material onto the ball. The wear tracks seen on the composite samples were both narrower and shallower, with substantially less debris transfer to the balls. The authors then used 3D optical surface profilometry to estimate the wear rate of each sample. The addition of fillers significantly enhanced the wear resistance (compared to pristine SMPU). For SMPU-MAX1 (0.25 wt%), the enhancement was ~50 times. For SMPU-MAX2 (0.25 wt%), it was ~100 times, and for SMPU-MXene (0.25 wt%), it was 500 times. From this, they concluded that “…as a filler material for PU, the Ti3C2 MXene phase material is superior to MAX phase material for tribological applications.” Elemental analysis of the wear tracks also revealed traces of constituent elements of MAX/MXene and SMPU (e.g. Ti, Al, C, and O) suggesting that these elements likely contributed to the low sliding friction they’d measured.
Finally, the authors tested the self-healing capabilities of the samples through the use of a universal tensile machine to intentionally dent the sample surface. Pristine SMPU contains an abundance of reversible physical crosslinks and thermo-reversible bonds. When an external stimulus (e.g. heat energy) is applied to a damaged piece of SMPU, these polymer chains rapidly diffuse, allowing it to re-form or ‘heal’. Their analysis showed that all three of the composites retained this ability. They conclude, “The results suggest that by using these composites, not only the friction and wear but also the frequent replacement of sliding components can be minimized, which is crucial for cost-saving and environmentally sustainable technologies.”
---
Shubham Jaiswal, Jeet Vishwakarma, Shubham Bhatt, Reuben J. Yeo, Rahul Mishra, Chetna Dhand, Neeraj Dwivedi. “Enhancing the lubricity and wear resistance of shape-memory-polymer via titanium carbide-based MAX and MXene,” Carbon 219 (2024) 118790. DOI: 10.1016/j.carbon.2024.118790